Hello,
In http://ceph.com/docs/next/rbd/rbd-config-ref/ it is said that:
"The kernel driver for Ceph block devices can use the Linux page cache to
improve performance."
Is there anywhere that provides more details about this?
As in, "can" implies that it might need to be enabled somewhere, somehow.
Are there any parameters that influence it like the ones on that page
mentioned above (which I assume are only for the user space aka librbd)?
That's misleading, when used via the kernel it behaves just like a normal block device (so, no need to turn on the page cache as it's just part of the system).
Also on that page we read:
"Since the cache is local to the client, there’s no coherency if there are
others accesing the image. Running GFS or OCFS on top of RBD will not work
with caching enabled."
This leads me to believe that enabling the RBD cache (or having it enabled
somehow in the kernel module) would be a big no-no when it comes to
KVM/qemu live migrations, seeing how the native KVM disk caching also
needs to be disabled for it to work.
I'm not sure about this, as I'm not sure what KVM requires here. The kernel client is safe for the named systems, period, though -- it looks and behaves just like a normal block device. The problem with the userspace cache is that it makes the local node behave the same way as a disk's RAM cache would -- so if you tell something to be durable, it will be, but you don't get coherency between different mounts because tr cache is all local.
-Greg
Am I correct in that assumption?
Regards,
Christian
--
Christian Balzer Network/Systems Engineer
chibi@xxxxxxx Global OnLine Japan/Fusion Communications
http://www.gol.com/
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Software Engineer #42 @ http://inktank.com | http://ceph.com
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